In wiper blades of the present invention, the support element should assure a predetermined distribution of the wiper blade pressing force—often also called pressure—applied by the wiper arm against the window, over the entire wiping zone that the wiper blade sweeps across. Through an appropriate curvature of the unstressed support element—i.e. when the wiper blade is not resting against the window—the ends of the wiper strip, which is placed completely against the window during the operation of the wiper blade, are loaded in the direction of the window by the support element, which is then under stress, even when the curvature radii of spherically curved vehicle windows change in every wiper blade position. The curvature of the wiper blade must therefore be slightly sharper than the sharpest curvature measured in the wiping zone of the window to be wiped. The support element thus replaces the costly support bracket design that has two spring strips disposed in the wiper strip, which is the kind used in conventional wiper blades (DE-OS 15 05 357).
The invention is based on a wiper blade as generically defined by the independent claims. In a known wiper blade of this type (DE-PS 12 47 161), a number of embodiments of the support elements are provided as a solution to the problem of producing the most uniform possible pressure load of the wiper blade over its entire length against a flat window.
In another known wiper blade of this generic type (EP 0 528 643 B1), in order to produce a uniform pressure load of the wiper blade against spherically curved windows, the pressure load increases significantly in the two end sections when the wiper blade is pressed against a flat window.
The uniform pressure distribution over the entire wiper blade length that is sought in both cases, however, leads to an abrupt flipping over of the wiper lip, which belongs to the wiper blade and performs the actual wiping function, over its entire length, from its one drag position into its other drag position when the wiper blade reverses its working direction. This drag position is essential for an effective, quiet operation of the wiper system. The abrupt flipping over of the wiper lip, however,—which is inevitably connected with an up and down motion of the wiper blade—generates an undesirable tapping noise. In addition, the matching of the support element tension to the desired pressure distribution, which differs from case to case, is problematic with spherically curved windows.
EP 0 594 451 describes flat bar wiper blades with a varying profile, which should not to exceed a particular lateral deflection when a test force is applied to them. To that end, an extremely complex interrelationship among internal parameters that characterize the spring bar are used to determine a quantity which should not exceed a certain threshold value. The equation given permits only complex and incomplete conclusions to be reached regarding the actual quantities to be entered. The other data relate to an unstressed wiper blade so that it is hardly possible to draw conclusions as to the quality of a wiper blade during operation.
In addition, putting the teaching of the known prior art to use turns out to be difficult since the available parameters cannot be applied directly to wiper blades to be newly manufactured.